REARRANGEMENTS OF ORGANOMETALLIC COMPOUNDS .16. STEREOCHEMICAL FACTORS IN THE CARBOMAGNESIATION OF UNSATURATED ALCOHOLS

In order to ascertain the stereochemical course of carbomagnesiating unsaturated alcohols, allylmagnesium bromide in ether was allowed to react, in turn, with 2-eyelopentenol, 3-cyclopentenol, 1-(1-propynyl)cyclohexanol, 1-(2-butynyl)cyclohexanol, and (2-cyclohexenyl)diphenylcarbinol. 2-Cyclopentenol and 1-(1-propynyl)cyclohexanol underwent substitutive allylation to yield, respectively, allylcyclopentenes and (2-allyl-2-methylvinylidene)cyclohexane. 3-Cyclopentenol gave an 80: 20 mixture of cis- and trans-3-allylcyclopentanol and some 3-(5-hexenyl)cyclopentanol and its dehydration product. In contrast, diallylmagnesium gave only cis-3-allylcyclopentanol and the hexenyl derivatives. l-(2-Butynyl)cyclohexanol gave only 1-[(+)-3-methyl-2, 5-hexadienyl]cyclohexanol, the product of cis carbomagnesiation; (2-cyclohexenyl)diphenylcarbinol gave the carbomagnesiated product, which had formed by syn addition to the olefinic bond from the side cis with respect to the hydroxydiphenylmethyl group. This last stereochemical relationship was shown by an NMR analysis on the lactone and the tert-butoxy derivatives formed from the Grignard adduct. In light of these findings, the fostering and orientating effect of the hydroxyl group on carbomagnesiation is ascribed to the formation and intramolecular rearrangement of an allylmagnesium alkoxide. The proximity of the allyl-magnesium bond to the carbon-carbon π bond brings about an electrophilic attack by the magnesium center and a net syn addition. © 1979, American Chemical Society. All rights reserved.